Scientists used to think that, among backboned animals, internal fertilization and carrying the young inside the mother's body originated in sharks and their kin some 350 million years ago. Before then, sexual reproduction in fish consisted of spawning, wherein females deposit eggs in the water, the males fertilize them, and the embryos then develop out in the open. Or so the story went.
As the cover story of the January 2011 Scientific American explains, recent analyses of fossils found in a remote locale in northwestern Australia and elsewhere have shown that intercourse and live birth actually arose millions of years earlier than previously believed—and in a more primitive group of fish than the one to which sharks belong. These fish—called placoderms—reside on the long line of animals leading us, and their sexual equipment gave rise to our own reproductive system and other parts of our anatomy.
In this video paleontologist John A. Long of the Natural History Museum of Los Angeles County, author of the cover story, discusses a fossil central to this new view of the origin of copulation and live birth: a 375-million-year-old expectant mother fish dubbed Materpiscis attenboroughi.